1. Field of the Invention
The present invention relates generally to an electrical erasable and programmable read-only memory (referred to as EEPROM hereinafter) and a manufacturing method therefor, particularly to reducing the programming voltages therefor.
2. Description of the Prior Art
In the description of the prior art, the U.S. Pat. No. 4,099,196 issued Jul. 4, 1978, entitled "Triple Layer Polysilicon Cell", assigned to Intel Corporation is incorporated by reference.
FIG. 1 is a cross sectional view showing a single cell of a conventional EEPROM having a triple gate structure which is disclosed in the above mentioned U.S. Pat. No. 4,099,196.
The EEPROM having a triple gate structure comprises a first polysilicon (polycrystalline silicon) gate electrode 3 formed on a silicon substrate 1 through a first gate oxide film 2 for writing information charge, a second polysilicon gate electrode 5 formed on the first polysilicon gate electrode 3 through a first polysilicon gate oxide film 4 and serving as a floating gate electrode, a third polysilicon gate electrode 7 formed on the second polysilicon gate electrode 5 through a second polysilicon gate oxide film 6 to overlap with at least a portion of the second polysilicon gate electrode 5 for controlling writing and erasing of the information charge, and a fourth polysilicon gate electrode 8 for erasing the information charge.
Operation is now described. When data is written, the EEPROM having a triple gate structure applies a voltage of approximately 25 V to the third polysilicon gate electrode 7 and the fourth polysilicon gate electrode 8, and the first polysilicon gate electrode 3 is grounded. As a result, electrons are injected from the first polysilicon gate electrode 3 to the second polysilicon gate electrode 5 by tunneling. In addition, when data is erased, the first polysilicon gate electrode 3 and the third polysilicon gate electrode 7 are grounded, and a voltage of approximately 25 V is applied to the fourth polysilicon gate electrode 8. As a result, electrons deposited on the second polysilicon gate electrode 5 are injected to the fourth polysilicon gate electrode 8 by tunneling. The oxide films 4 and 6 on the first polysilicon gate electrode 3 and the second polysilicon gate electrode 5 are approximately 1000.ANG.. Since electrons tunnel through the oxide films 4 and 6 at approximately 25 V, the EEPROM having a triple gate structure is formed such that a portion from which information charge is emitted, of the polysilicon layers 3 and 5 serving as electrodes, has a rough surface. The rough surface allows tunneling of electrons even at a relatively low voltage.
Conventionally, the following approaches have been employed so that the surface of the portion from which information charge is emitted, of the first polysilicon gate electrode 3 and the second polysilicon gate electrode 5 easily becomes rough.
(1) A first polysilicon gate oxide film is formed by thermal oxidation at a low temperature. (For the fact that the surface of polysilicon becomes rough more easily by thermal oxidation at a low temperature, see an article by R. M. Anderson et al., entitled "Evidence for surface asperity mechanism of conductivity in oxide grown on polycrystalline silicon", J. of Applied Physics, Vol. 48, No. 11, November, 1977.)
(2) The concentration of implanting conductive impurities into polysilicon serving as a first polysilicon gate and a second polysilicon gate is decreased.
Description is now made on a method for manufacturing the EEPROM. FIGS. 2A to 2E are diagrams showing the sequential steps of the manufacturing method for the conventional EEPROM.
A p type silicon substrate 1 is prepared (FIG. 2A). A first gate oxide film 2 is then formed on the silicon substrate 1 (FIG. 2B). A polysilicon layer 8 with the concentration of conductive impurities decreased is then formed on the first gate oxide film 2 to be a first polysilicon layer 3 (FIG. 2C). A first polysilicon gate oxide film 4 is formed in the upper and side portions of the first polysilicon layer 3 by thermal oxidation, and a second polysilicon gate electrode 5 with the impurity concentration decreased is formed thereon (FIG. 2D). A second polysilicon gate oxide film 6 is formed in the upper and side portions of the second polysilicon gate electrode 5 by thermal oxidation, and a third polysilicon gate electrode 7 and a fourth polysilicon gate electrode 8 are formed thereon.
The manufacturing method for the conventional EEPROM having a triple gate structure comprises the steps of, for example, decreasing the concentration of impurities contained in polysilicon and decreasing the thermal oxidation temperature of a gate oxide film so that the first polysilicon gate electrode and the second polysilicon gate electrode have rough surfaces. However, it is difficult to make uniform the concentration of impurities contained in polysilicon within the surface of the silicon substrate. Therefore, the rough surfaces of the polysilicon cannot be made uniform. Thus, electric characteristics are not kept constant, yield of the EEPROM is decreased and the reliability is decreased.